I have this exact problem in my '01 ZX3, I get up to highway speeds and i get this kind of clunking noise coming from the drivers side, i even asked my dad for any ideas as to what it could be seeing as he usually knows if i don't and he's just as stumped as I am.

well if its waiting for you to get up to highway speeds... then you need to check your tires AND alignment... i had this issue with my 02 zx5... the alignment was/is off, and it chews the tires out of the outside causing the vibration... you could also check the wheel bearings (all of them) as i have also had this issue...

again, try checking your alignment and wheel bearings if the tread is okay (check particular on the outside and inside edges of the tires of scalloping... that will created an insane wobble/vibration/hum at 55mph and plus...